It is well known to provide track movable blocks for marine use. In such case, the track may be mounted on a boat deck. The movable block is provided with a slide portion which engages the track edges, the block being pivotally attached to the slide portion. In the case of a lead block, it is often desirable to orient the track for fore and aft movement of the block, the latter pivoting crosswise or sideways with respect to the track orientation. In operation, a sheet is fed into the block around the block sheave, and pivotability of the block readily permits it to adapt to the bias of the sheet.
However, it frequently is necessary to quickly transfer a sheet from one block to another, for instance in racing, or introduce a different sheet into the block, and depending upon the length of the sheet, an inordinate amount of time may be required to unthread a sheet from a block and then go through the step of rethreading it or a different sheet. In racing in particular, a need exists for a track mounted snatch block which eliminates the unthreading and rethreading steps required of conventional blocks. At the same time, the block must be simple in construction, capable of resisting corrosion for instance by exposure to salt water, and must have a high strength-to-weight ratio for obvious reasons.
Snatch blocks are well known and have been developed for a variety of purposes. An example of one such block is shown in prior U.S. Pat. No. 2,555,059 to Schrader. In this patent, there is illustrated a swingably mounted block side which swivels on the same journal pin on which the block sheave or pulley is mounted, the side swinging or pivoting in a plane perpendicular to the pin or parallel to the line of force normally applied to the block. The side has an inwardly extending hook shaped extension which engages a recess in the block when the side is swung to a closed position. To move the side from a closed to open position, the side has on its exposed face an enlargement called in the patent a finger rest suitable for hand pressure. It is anticipated that the design of this patent would be subject to fatigue cracks under high loads by virtue of the manner of engagement between the swingable side and the body of the block. In addition, a block of this type suffers in the limited amount of access to the sheave provided by the swingable side. Such a block would be totally unsuitable for marine use. Particularly at night, it would be a difficult matter to rapidly load such a block during a lead change.
Prior U.S. Pat. No. 3,199,841 to McKean is illustrative of a different type of snatch block. This patent discloses a block housing in which the sheave cover pivots between an open and closed position exposing the sheave groove. One problem with the design of this patent is that it is not readily adaptable to track mounting. The cover itself would have to be integral with or fastened to the track slide, and then the entire block assembly including the sheave and sides would have to pivot relative to the sheave cover. Since both sides would be conveyed with the sheave, in the pivoting movement, it would be necessary, in threading or unthreading a sheet from the block, to pull the sheet around the free edge of the side which is pivoted away from the sheave cover. This can be awkward, particularly at night, rendering the design of the McKean patent unsuitable for marine use.